In the past the sidewall structure of cargo carrying vans and trailers is constructed having a typical design and width. The useable capacity of the van or trailer is dependent in part on the width of the opposed sidewall assemblies. The width of long used sidewalls on a van and the like is at least 1.5 inches and comprises a plurality of hat-shaped side posts to which outer and inside side sheets which are riveted by a series of vertically aligned rivets. Attempts have been made to reduce the thickness of the standard sidewalls so that useable space of a van or trailer can be increased. An example of one prior art design for reducing sidewall thickness comprises a thick aluminum plate to which a shallow shaped beam is riveted at the vertical interface between adjacent plates. Such a known plate construction is capable of reducing the thickness of each opposed sidewall structure to approximately ⅝ inch. Other prior designs have also attempted to reduce the thickness of the sidewall by employing a pair of thin metal plates sandwiched over a polyethylene material to form a composite structure having no side posts or beams. The foregoing techniques present several disadvantages including higher weight repair costs, reduced number of logistic posts required for securing loads, or produce a highly flexible structure which may be unsuitable under all loading conditions. All of the known sidewall assemblies further require the extensive use of mechanical fasteners, such as rivets, that results in uneconomical labor costs and time in assembly as well as contributing to additional weight of the sidewalls. Accordingly, it is desirable in the prior art to provide sidewall assemblies for vans and trailers having an optimally thin cross section and which reduce cost of fabrication and assembly and maintains the weight compared to today's standards.